Club root (Plasmodiophora brassicae)
This fungus is classified into a quite separate group of fungi, the
It causes serious damage to most members of the Cruciferae
family, which includes cabbage, cauliflowers, Brussels sprouts, stocks
and Alyssum. Infected plants show signs of wilting and yellowing of
older leaves, and often severe stunting. On examination, the roots appear
stubby and swollen (see Figure 15.12), and may show a wet rot.
Life cycle and spread:
|Figure 15.12 Club root on cabbage
The club root organism survives in the soil for
more than five years as minute spores which germinate to infect the root hairs of susceptible plants. The fungus is unusual in forming a jelly-like
) , not hyphae, within the plant’s root tissues. The
plasmodium stimulates root cell division and causes cell enlargement,
which produces swollen roots. The flow of food and nutrients in phloem
and xylem is disturbed, with consequent poor growth of the plant. With
plant maturity the spores produced by the plasmodium within the root
are released as the root rots.
The disease is favoured by high soil moisture, high soil temperatures and
acid soils. Although this fungus does not spread much in undisturbed
soils it can be easily carried on infected plants, or on tools and wheels of
machinery. In peat soil-growing areas, high winds may carry the disease
a considerable distance.
Several preventative control measures may be used by the amateur
gardener and professional
grower. Rotation greatly helps
by keeping cruciferous crops away from high spore levels in the
soil. Liming of soil greatly inhibits spore activity (see soil fertility
Recently released cultivars of late-summer cabbage are claimed to have
strong resistance to club root. Autumn-sown plants establish in soil
temperatures unfavourable to the disease and are normally less infected.
Compost made from infected brassica plants should be avoided. The professional
grower using transplants can prepare a seedbed previously
sterilized with a granular product containing dazomet,
ensure healthy transplants.
Damping off (Pythium and Phytophthora species)
|Figure 15.13 Damping off on seedlings. Note the
shrivelled, papery appearance of the leaves
on the infected
These two fungi belong to the Zygomycota.
These two similar genera of fungi cause
considerable losses to the delicate seedling
infection may occur below the soil surface, but most
commonly the emerging seedling plumule is infected at
the soil surface, causing it to topple (see Figure 15.13).
Occasionally the roots of mature plants, e.g. cucumbers,
are infected, turn brown and soggy, and the plants die.
Rose plants often have high levels of Pythium
their roots as they age. Although the mature plant is not
seriously affected, it is a common experience that on
removal of the plant and replacement with a young rose
plant, there is a quite rapid decline in its vigour, called rose-sickness
Life cycle and spread:
occur naturally in soils as saprophytes, but under damp
conditions they produce the asexual spores that cause
infection. These spores are spread by water. Sexual spores
(oospores) are produced in infected roots (mostly in
autumn) and may survive several months of dry or cold soil
Prevention control is best achieved (both for the amateur
gardener and the professional
grower) against these diseases by providing a disease-free growing medium. This may
be produced by using fresh compost, by partial sterilization of soil with
heat, or (for the professional
grower) by a sterilant such as dazomet
Seed producers often coat crop seed with a protective seed dressing (see
also) such as thiram
to prevent early infection.
Water tanks with open tops, harbouring rotting leaves, are a common
source of infected water and should be cleaned out regularly. Sand and
capillary matting on benches in greenhouses should be regularly washed
with hot water. The use of door mats soaked in a sterilant such as dilute formalin
may prevent foot spread of the organisms from one greenhouse
to another. Waterlogged soils should be avoided, as these fungi increase
most rapidly under these conditions. The amateur
gardener may use a copper
formulation (known as Cheshunt mixture) as a drench to slow
down the increase of damping off. Professional
growers use a product
, which may be mixed in with composts, or
drenched on to seed trays, pots or border soil growing young plants.
|Figure 15.14 Conifer root rot. Note the different
of colour in
different stages of infection
Conifer root rot (Phytophthora cinnamomi)
This fungus belongs to the Zygomycota group of fungi.
This soil-inhabiting fungus is most commonly a problem in
nursery stock production nurseries. It causes the foliage of plants to
turn grey-green, then brown and eventually to die off completely (see
Figure 15.14). Sliced roots show a chestnut brown rot, with a clear line
between infected and non-infected tissues. Two hundred plant species,
including Chaemaecyparis, Erica and Rhododendron
species may be
Life cycle and spread:
The disease is commonly introduced on infected
stock plants or contaminated footwear. It multiplies most rapidly under
wet conditions, within a temperature range of 20°C and 30°C, infecting
the root tissues and producing numerous asexual spores, which may be
spread by water currents to adjacent plants. Sexual oospores produced
further inside the root are released on decay and allow the fungus to
survive in the soil for several months without a host.
Preventative control (see hygienic growing
important. Reliable stock plants should be used. Water supply should be
checked to avoid contamination. The stock plant area should be elevated
slightly higher than the production area to prevent infection by drainage
water. Rooting trays, compost and equipment, e.g. knives and spades,
should be sterilized (e.g. with formalin) before use. Placing container
plants on gravel reduces infection through the base of the pot. The
incorporated in compost protects the roots, but
does not kill the fungus. Some species, such as Juniperus horizontalis,
have some tolerance to this disease.
Honey fungus (Armillaria mellea)
This fungus belongs to the Basidiomycota group of fungi.
|Figure 15.15 Honey fungus. Note the dense clump of
honey-coloured toadstools, and also
strands (rhizomorphs) spreading out from the clump.
This fungus primarily attacks trees and shrubs, e.g. apple, lilac
and privet. In spring the foliage wilts and turns yellow. Death of the
plant may take a few weeks or several years in large trees. Confirming
symptoms are the white mycelium, rhizomorphs and toadstools
mentioned below (see Figure 15.15).
Life cycle and spread:
The infection process involves rhizomorphs
(sometimes referred to as ‘bootlaces’), which radiate out underground
from infected trees or stumps for a distance of 7 m, to a depth of 0.7 m.
The infected stump may remain a serious source of infection for twenty
years or more. The rhizomorphs are the only means of spread for this
disease. The nutrients they are able to conduct provide the considerable
energy required for the infection of the tough, woody roots. Mycelium,
moves up the stem beneath
the bark to a height of several metres and
is visible (when the bark is pulled away) as white sheets, smelling of
mushrooms. In autumn, clumps of light-brown toadstools
produced, often at the base of the stem. The millions of spores produced
by the toadstools are not considered to be important in the infection
process. Honey fungus often establishes itself in newly planted trees and
shrubs that have been planted too deeply. Deep planting produces less
vigorous plants that are more vulnerable to infection. Vigour is reduced
because feeding roots which ideally should be growing near the surface
of the soil have been located in the subsoil.
is difficult. Some genera of plants are less likely to be infected
(see Table 15.1). Removal of the disease source, the infected stump,
is strongly recommended. In large stumps which are hard to remove a
surrounding trench is sometimes dug to a depth of 0.7 m to prevent the progress of rhizomorphs. Loosening soil with a fork and then applying
a sterilant, e.g. formalin
in a diluted state, may be applied in situations
where there are no crops.
Fusarium patch on turf (now called Microdochium
|Table 15.1 Levels of resistance to Honey fungus in garden shrubs and trees
This belongs to the Deuteromycota group of fungi.
|Figure 15.16 Fusarium patch on turf.
Note the area of
dying turf. In
earlier stages of the disease,
circular patches about 30 cm
across are seen.
This disease appears as irregular circular patches of yellow
then dead brown grass up to 30 cm in diameter on fine turf. These
patches eventually merge (see Figure 15.16 ). Under
extreme damp conditions, dead leaves become
slimy and then are covered with a light pink bloom,
most evident between May and September.
Life cycle and spread:
Infection of the leaves
by spores and hyphae occurs most seriously
between 0°C and 8°C, conditions that are found
under a layer of snow (hence its other name, snow
). However, conditions of high humidity at
temperatures up to 18°C may result in typical patch
symptoms. Spread is by means of water-borne
asexual spores under conditions such as autumn
dew with no wind. The fungus can survive in frosty
or dry summer conditions as dormant mycelium in
dead leaf matter or newly infected leaves.
Preventative control measures are
important. Avoid high soil nitrogen levels in autumn, as this promotes
lush, susceptible growth in autumn and winter. Avoid thatchy growth of
the turf, as this encourages high humidity and thus favours the disease
organism. The groundsman can drench preventative fungicide such as iprodione
in autumn to slow down infection of the fungus. Summerapplied
systemic fungicide such as thiophanate methyl
is able during
the actively growing period of the year to move within the plants and
achieve curative control.
Vascular wilt diseases (Fusarium oxysporum and
These fungi belong to the Deuteromycota group of fungi.
|Figure 15.17 Fusarium wilt on beans.
These two organisms infect the xylem
tissues of horticultural
plants, causing the leaves to wilt in hot conditions, a symptom which can
also be caused by other factors, e.g. lack of soil moisture (see wilt) and
nematode infestation (see root knot nematode). The wilt diseases
can be recognized by yellowing and eventual browning of the lower
leaves (see Figure 15.17) and by brown staining of the xylem tissue when
it is exposed with a knife. Verticillium
may attack a wide range of plants,
e.g. dahlia, strawberry, lilac, tomato and potato, so that rotation is not a
feasible control measure. Fusarium oxysporum
, however, exists in many distinct forms, each specializing in crops in different plant
families, e.g. tomato, cucumber, bean or carnation.
Life cycle and spread:
Both organisms may live as saprophytes
in the soil. Fusarium
conditions as thick-walled asexual spores, while Verticillium
forms small sclerotia. Infection by both genera
occurs through young roots or after nematode attack
in older roots. The fungal hyphae enter the root xylem
tissue and then move up the stem, sometimes reaching the
flowers and seeds. The diseases are spread by water-borne
asexual spores. The two fungi have different temperature
more commonly attacks in
springtime, having an optimum infection temperature of
20°C, while Fusarium
is more common in summer, with
an optimum temperature of 28°C.
Control is often necessary in greenhouse crops.
Infected crop residues should be carefully removed from the
soil at the end of the growing season. The amateur
grower may choose to use peat bags instead
of soil. The professional may use partial soil sterilization by
steam, or a chemical sterilant such as metam-sodium:
In unsterilized soils, professional
growers may use resistant rootstocks,
e.g. in tomatoes, which are grafted onto scions of commercial cultivars.
Rotation may be employed against a Fusarium oxysporum attack, as
different forms attack different crops. Careful removal of infected and
surrounding plants, e.g. in carnations, may slow down the progress of the
diseases, especially if the soil area is drenched with a systemic chemical
such as carbendazin
, which reduces the infection in adjacent plants.